Traditionally, a microprocessor's average current draw is monitored by converting analog values of current to digital values that can be used by the microprocessor. This required some form of analog-to-digital (A/D) converter and an averaging circuit. The standard method of measuring current involves sending the current through a known “sense” resistance and measuring the voltage drop across the resistor. The known resistance and measured voltage can be plugged into Ohm's law (I=V/R) to calculate the current.
This method involves the design of an A/D converter for measurement of the voltages on both sides of the sense resistor. Using a discrete, off-die A/D converter to measure the voltage drop requires a separate component that is on or very near the microprocessor package, which increases the microprocessor package cost. Furthermore a separate interface is needed between the microprocessor and the A/D converter, which further complicates the design.
An alternative design is to put the A/D converter into the package, which would be a very complicated process and is likely to yield relatively inaccurate results in a digital integrated circuit manufacturing process by which the microprocessor is fabricated. The addition of the sense resistor also wastes power by dissipating power in the resistor instead of putting it to useful work in the circuit.